Document read replication and transaction behaviour

[Rochet2]

It appears that there is no documentation that I could find on read replication and how useMaster is not needed for select queries in a transaction for consistency. This PR should document the default behaviour of SELECT queries in a transaction when using read replication.

In V7 there will be a new setting for transactions to allow going to read pool only. It could also be mentioned here maybe?

I also wonder (have not tested) what happens if one explicitly sets useMaster true/false inside a transaction. Maybe something to document as well.